Well, I don't know for sure, but I wonder if the statement is even correct.
How often do you see a piano where the player face the audience? In most cases people encounter real pianos (ie at home) the piano faces the wall and the player has his/her back to the audience. In concert situations the pianist usually sits with his side to the audience, so there is no distinctive "panning" at all.
So people "know" that the bass is to the left on a piano, because that's how they usually hear/see it in real life. Plus, most people has plonked on a piano at some time. While few people has played the drums, and mostly see drumsets in rock/jazz settings, with the drummer facing the crowd, hence people "know" that the hi-hat should be to the right, and the tom to the left, even if it's the other way around for the drummer.
So I'd claim that both drums and pianos are usually panned to the listeners perspective.